Dislocation of the hip joint is a major cause of failure in hip arthroplasty. The reported incidences of hip dislocations range from 0.5% to 5.8% for primary surgeries and from 4.8% to 13% for revision surgeries. (See Burroughs et al. “Range Of Motion And Stability In Total Hip Arthroplasty With 28-, 32-, 38-, and 44-mm Femoral Head Sizes,” J Arthroplasty 2005 January, 20(1):11-9.) Additionally, a large proportion (30% to 65%) of these dislocations become recurrent. (See previously mentioned Burroughs et al.) To reduce the risk against hip dislocations, large diameter (greater than about 32 mm) femoral heads and mating acetabular components are used in a variety of hip arthroplasty implants such as total hip implants, resurfacing hip implants, and dual mobility (DM) hip implants.
As shown in FIG. 1, a conventional total hip implant is generally composed of an acetabular shell 10 that mounts on the native pelvic bone and replaces the native acetabulum, an acetabular liner 12 affixed immovably to the acetabular shell 10, a prosthetic femoral head 14 that replaces the native femoral head, and a femoral stem 16 that attaches to the prosthetic femoral head 14 via a femoral neck 18. Some hip arthroplasty implants, such as metal-on-metal implants and resurfacing implants, do not have an acetabular liner, and the femoral head articulates directly with the acetabular shell. FIGS. 2A and 2B illustrate conventional large diameter femoral heads used in hip resurfacing (FIG. 2A) and total hip implants (FIG. 2B). The femoral head diameter equals 2*R, where R is a radius of an overall spherical geometry 22 of the femoral heads. Prosthetic femoral heads with diameters of about 32 mm or greater are defined as large diameter, and this diameter is close to that of the patient's native femoral head. In contrast, femoral heads with diameters of about 28 mm or less are defined as small diameter, and this diameter is smaller than that of the native femoral head.
As shown in FIGS. 3A and 3B, a conventional dual mobility hip implant is generally composed of an acetabular shell 24, a mobile insert 26, and a small diameter femoral head 28 attached to a femoral stem 30. The acetabular shell 24 mounts on the native pelvic bone and replaces the native acetabulum, and the femoral stem 30 attaches to the femoral head 28 via a femoral neck 32. An outer surface of the mobile insert 26 articulates with the acetabular shell 24 to form an outer articulation. An inner surface of the mobile insert 26 articulates with the small diameter head 28 retained within the insert 26 to form an inner articulation. An outer diameter of the mobile insert 26 is typically about 36 mm or greater. In contrast, the femoral head 28 retained within the mobile insert 26 typically has a diameter of about 28 mm or less. The larger diameter (2*R) outer articulation between the acetabular shell 24 and the mobile insert 26 provides stability against dislocation and provides large range of motion. The small diameter (2*Ri) inner articulation between the mobile insert 26 and the femoral head 28 provides a low wear articulation. Extraction or dislocation of the small diameter head 28 from the inner articulation is prevented by retention of the head 28 within the mobile insert 26. This retention is achieved by having the inner articular surface of the mobile insert 26 designed to cover and capture more than a hemispherical portion of the femoral head. Parameter β in FIGS. 2A and 2B characterizes an angular extent of an outer articular surface 30 of a femoral head, and parameter β in FIG. 4 characterizes an angular extent of an outer articular surface 32 of a mobile insert.
While large diameter femoral heads and mobile inserts provide increased resistance to hip dislocation, one of the concerns with conventional designs is the potential impingement against native soft tissues, such as the hip capsule and the iliopsoas muscle/tendon (see FIGS. 5A, 5B, 6A, and 6B). Impingement of these soft tissues can lead to severe groin pain. FIG. 5A shows that the iliopsoas tendon 34 in the native hip passes over the native femoral head 36 and femoral neck 38 to insert into the lesser trochanter 40. FIG. 5B shows that the iliopsoas tendon 42 articulating against the native femoral head 44 in a cadaver human hip joint (see Yoshio et. al. “The Function Of The Psoas Major Muscle: Passive Kinetics And Morphological Studies Using Donated Cadavers,” J Orthop Sci. 2002, 7:199-207). Arrow 43 indicates a location of iliopsoas articulation against the native femoral head 44. FIGS. 6A and 6B show a conventional large diameter femoral head or mobile insert 46 mounted on a computer tomography (CT) based bone model of a cadaver specimen including a femur 47 and a pelvis 49. The articular surface of the prosthetic femoral head or mobile insert can be seen in FIGS. 6A and 6B to overhang 48 the articular surface of the native femoral head, particularly in the anterior-distal/anterior-medial and posterior-distal/posterior-medial regions. Acetabular shells and acetabular liners have been proposed to address potential soft tissue impingement. (See US Pat. Pub. No. 2005/0060040 filed Sep. 9, 2004 entitled “Prosthetic Acetabular Cup And Prosthetic Femoral Joint Incorporating Such A Cup,” Intl. Pat. No. WO 2009118673 filed Mar. 20, 2009 entitled “Cotyloidal Prosthesis Of The So-Called ‘Dual Mobility’ Type,” US Pat. Pub. No. 2011/0301654 filed Jul. 29, 2011 entitled “Hip Resurfacing,” and U.S. Pat. No. 7,169,186 filed May 15, 2002 entitled “Monopolar Constrained Acetabular Component.”)
Another complication relating to use of large diameter femoral heads and mobile inserts is increased wear and/or frictional torque at the femoral head-acetabular articulation or mobile insert-acetabular articulation. (See Lachiewicz et al. “Femoral Head Size and Wear of Highly Cross-linked Polyethylene at 5 to 8 Years,” Clin Orthop Relat Res. 2009 December, 467(12): 3290-3296; Livermore et al. “Effect of Femoral Head Size on Wear of the Polyethylene Acetabular Component,” J Bone Joint Surg Am. 1990 April, 72-A: 518-528.)
Yet another complication of using large diameter femoral heads and mobile inserts is increased risk of failure of the modular junctions used in many modern hip implants. Such modular junctions are used for increased flexibility in matching a patient's anatomy and achieving optimal component positioning (see, e.g., a modular taper junction 20 of FIG. 1). A modular femoral head-neck junction, for example, implies that the femoral head is a separate component that is assembled onto the femoral neck. As shown in FIGS. 7A and 7B, conventional modular junctions of a femoral head 52 and a femoral neck 54 are typically conical taper junctions 50, with a small diameter circular profile d1 at one end of the taper junction 50 that increases in size to a circular profile of diameter d2 at an opposite end of the taper junction 50. A parameter L represents a length of the taper junction 50 measured along a taper junction axis 50A, and a parameter λ represents a conical angle of mating surfaces of the taper junction 50. In a conventional femoral head-neck taper junction, the taper junction axis is generally also parallel to the prosthetic femoral neck axis, as in FIGS. 7A and 7B.
Recent studies have shown that taper junctions are susceptible to corrosion due to micromotion at the mating surfaces. (See Lieberman et al. “An Analysis Of The Head-Neck Taper Interface In Retrieved Hip Prostheses,” Clin Orthop Relat Res. 1994 March, (300):162-7; Rehmer et al. “Influence Of Assembly Procedure And Material Combination On The Strength Of The Taper Connection At The Head-Neck Junction Of Modular Hip Endoprostheses,” Clin Biomech. 2012 January, 27(1):77-83.) This in turn can lead to loosening of the modular junction and create undesirable metal debris. Hip implants with large diameter femoral heads and mobile inserts are particularly susceptible to this due to the increased lever arm and implant diameter, which can lead to greater frictional torque and moment loads at the junction. (See Meyer et al. “Corrosion at the Cone/Taper Interface Leads to Failure of Large-diameter Metal-on-metal Total Hip Arthroplasties,” Clin Orthop Relat Res. 2012 Aug. 3. [Epub ahead of print]; Langton et al. “Taper Junction Failure In Large-Diameter Metal-On-Metal Bearings”. Bone Joint Res. 2012 September, 1(4): 56-63.)
Accordingly, there remains a need for improved orthopedic implants.